This invention generally refers to modulation control circuits and more specifically to deviation control circuits for digital frequency or phase modulated transmitters useful in narrow band land mobile communications frequencies.
The channels available for land mobile communications are typically spaced 25 KHz or 30 KHz apart in the UHF and VHF bands such that the transmissions from one transmitter do not interfere with the transmissions of a transmitter on an adjacent channel. Generally, frequency (or phase) modulation is used on these channels to convey analog information such as voice from a transmitter to a receiver. It is also desirable to transmit data over the same channels. However, when high bit rate signals are modulated onto a radio frequency (RF) carrier, a radio spectrum much wider than that produced by analog signals is generated. This wide spectrum overlaps or splatters energy into adjacent channels and can result in poor system sensitivity for a receiver tuned to the adjacent channel. Therefore, in order to avoid splatter it is the task of a transmitter modulator to frequency translate a digital baseband signal to a radio frequency signal such that the modulation energy in a 10 KHz wide channel centered 25 KHz from the RF carrier of the signal is at least 60 dB below the level of the carrier. It is also desirable that the digital baseband signal be as high a bit rate as possible.
Several different modulation techniques have been employed for narrow bandwidth transmissions. One type is offset quadrature phase shift keying (OQPSK) which instantaneously shifts the phase of the carrier by zero or plus or minus pi divided by two radians for each bit time. The pulse shape at the frequency modulator input is an impulse containing the exact area necessary to cause a pi divided by two phase shift and which results in an effectively infinite frequency deviation and unacceptably wide transmitted sprectrum. A second form of data modulation is called minimum shift keying (MSK) which modulates the carrier by instantaneouly shifting the carrier frequency. A digital 1 is represented by a positive shift in frequency such that the carrier phase changes by a positive pi divided 2 radians during the period of the bit time and a digital zero is represented by a negative shift in carrier phase such that the phase changes by a negative pi divided by two radians during the bit time. The pulse shape presented to the input of a frequency modulator is rectangular and does not present impulses generating a wide frequency spectrum. However, when the data changes polarity the second derivative of the waveform at that point results in impulses which cause unacceptably wide transmitter deviations. A third modulation technique is called sinusoidal frequency shift keying (SFSK) which is an attempt to eliminate the second derivative impulses present with MSK. SFSK accomplishes this by sinusoidally shaping the phase waveform input to the frequency modulator during each bit time rather than using the linear phase path of MSK. The SFSK phase path is smooth at the points where the data bit changes polarity and allows higher derivatives of the carrier phase waveform to exist without impulse responses. However, because the SFSK waveform shaping occurs during a bit time, the peak instaneous deviation is twice as large as MSK and the modulation frequency is changed even when the data polarity does not change. Therefore, a wide spectrum is created with SFSK.
Two types of modulation techniques, tamed frequency modulation (TFM) and premodulation Gaussian-filtered spectrum-manipulated minimum shift keying (GMSK), can result in reduced spectrum occupancy with high data bit rate. This bandwidth reduction is accomplished by allowing some interference between neighboring pulses in a precisely defined manner. In the case of TFM, the total phase change of the carrier during a bit time is determined by applying a correlation coding function which is also known as a partial response coding. These functions code a serial binary bit stream into a serial stream of multilevel symbols. The coding function used in TFM codes three consecutive binary digits into a five level bit stream which is modulated into different carrier phase changes over one symbol period. GMSK utilizes a precisely defined Gaussian filter prior to the input of the frequency modulator thereby reducing the spectrum occupancy of the modulated carrier while retaining enough information such that individual bits may be recovered at the receiver.
These last two modulation techniques, however, require that the carrier frequency and the modulation sensitivity be invariant. In realizable systems these parameters are insufficiently constant and require special measures to be taken to keep them at the prescribed values. For example, the modulation sensitivity should be maintained within .+-.2% of the design value. Several techniques have been suggested in the literature to overcome the modulation sensitivity instability problem. One such technique uses a phase locked loop which feeds back the square of the modulated data signal and locks to the two spectral lines which are a result of the squaring operation. A second uses a ROM look up table followed by a D-A converter which produces quadrature carrier signals which are subsequently fed to a quadrature modulator to produce the modulated signal. A third requires periodic calibration of a discriminator with carriers at the appropriate peak deviation. During transmission, the discriminator output is monitored and the modulator input level is adjusted to provide the proper deviation.
Each of these ways of solving the modulation sensitivity instability problem requires either a special relationship between frequencies of modulation, or added complexity of circuit in a quadrature modulator, or an added burden of calibration. The modulation cancellation and detection approach of the present invention avoids these problems and yields stable modulation sensitivity.